Windows are important and high-cost components of the exterior of a building. By reducing heat loss through single pane windows, domestic energy consumption in the United States can be reduced by an estimated 1.3%. Heat loss in windows can be decreased by increasing the thermal insulation of a window, which is commonly described by the U-factor (i.e., BTU ft−2 h−1° F.−1).
Present attempts to reduce heat loss through single pane windows include replacing them with double-paned insulated glass units (IGUs), which can incorporate inert gas insulation and/or low-emissivity coatings. While IGUs are generally effective at reducing heat loss, they are prohibitively expensive for retrofitting existing single pane windows (e.g., $50-100/ft2) because IGU installation requires the replacement of the window framing and sash, in addition to the pane.
In addition, IGUs are significantly heavier than single pane windows and they change the appearance of the window, both of which are factors that prevent their adoption for single pane window retrofits. Thus, the need exists for inexpensive, highly-insulating, transparent films or manufactured panes that can be retrofitted to single pane windows.
Optical transparency and low haze are generally of primary importance for window materials. High-thermal insulation and high-transparency are difficult to achieve simultaneously in a material. High porosity increases thermal insulation by reducing the amount of solid, high-thermal conductivity material, and disrupting convective heat transfer; however, pores that are above ˜10 nanometers scatter visible light, thus increasing the haze and decreasing the transparency of the material.
Many highly-insulating materials such as fiberglass, polyurethane foams, and silica aerogels are porous but opaque or translucent, thus rendering them unsuitable for window applications. For insulation materials to be transparent, porosity must be controlled and on the nanometer-length scale.
Conventional radical polymerization produces polymer gels with a large range of pore sizes, including some that are very large. This results in hazy and often white, opaque gels.
At the present time, there is no available process for fabrication of high surface area all-polymer aerogels dried by ambient solvent evaporation. Supercritical CO2 dried polyimide (PI) aerogels with a BET surface areas of up to about 500 m2/g have been reported. Supercritical drying prevents shrinkage and pore collapse by preventing the solvent from undergoing a direct liquid-to-vapor transition. However, supercritical drying is expensive, slow, and hazardous due to the high pressure required, and it is therefore not desirable for fabrication of large area (e.g., >15 sq. ft.) and for high-throughput manufacturing. Ambient drying is a key requirement for achieving low cost aerogel films.
As used herein, the term ambient drying generally refers to any process that results in direct solvent evaporation of the solvent, starting from a liquid in the gel into a vapor, and note that the applied air pressure and temperature may be different from that of the ambient air. Other alternatives to supercritical drying and ambient drying such as freeze drying also result in higher costs compared to ambient drying.
A challenge is that commercial ambient air dried PI aerogels have a much lower surface area, typically BET<100 m2/g. This is due to shrinking of the polymer network as a result of the capillary pressure arising during solvent exchange and/or drying in ambient conditions. Several properties require BET area>100 m2/g. For instance, achieving very low thermal conductivity requires high BET areas because high BET areas are generally due to high porosity and small pore sizes. This decreases the fraction of thermal conductivity transport through the aerogel skeleton, and pores smaller than the mean free path of the ambient gas (e.g. air) leading to the Knudsen effect, where the thermal conductivity can decrease below that of the ambient gas (e.g., air at 0.025 W/m K in typical conditions).
An even more challenging problem is related to the fabrication of low haze transparent polymer aerogels. PI aerogels are colored deep yellow and are opaque. High porosity increases thermal insulation by reducing the solid/air ratio, since the solid has higher thermal conductivity, suppressing convective heat transfer for pores smaller than <1 mm, and further decreasing thermal conductivity by the Knudsen effect when the pores are smaller than the mean free path of air (˜80 nm in typical conditions). However, pores that are larger than 20 nm scatter visible light, which increases the haze and decreases the transparency of the material. For aerogels to be transparent, the pore size must be controlled. Transparency and low haze require small pores (<20 nm) and narrow pore size distribution.
In order to achieve transparent aerogels, the polymer forming the aerogel structure must be transparent and colorless. Vinyl polymers are some of the most transparent polymer materials known today. For example, poly-methylmethacrylates and other acrylate derivatives have been used for fabrication of acrylic windows, for highly transparent optical adhesives, and for interlayers in laminated glazings. Therefore, they would be the ideal candidates for producing transparent aerogels. In addition to transparency they also are amenable to processes that enable <20 nm and narrow pore size distribution. However, the use of common acrylic monomers resulted in aerogels with poor porosity as indicated by BET area <50 m2/g. The capillary pressure induced aerogel shrinking becomes larger as the pore size of the aerogel decreases.
If available, transparent all-polymer aerogels would be useful as transparent insulation that could for example be used for highly insulating windows. Various windowpanes could be fabricated, including windowpanes that have dimensions identical to single pane windows, which would enable a new retrofit or replacement of single pane windows.
Reducing the heat loss through existing single pane windows could potentially lower US energy consumption by 1.3%. Currently, the heat loss through single pane windows is reduced by replacing them with double, triple, or quadruple pane windows, or insulated glass units (IGUs), which can incorporate inert gas insulation and, or low emissivity coatings. While IGUs are effective at reducing heat loss, they suffer several drawbacks such as high cost ($50-100/ft2) due to the replacement of the window framing and sash, high weight, and unaesthetic appearance which may be an important attribute for historic buildings. Other applications and markets where high transparency, highly insulating single panes are potentially valuable include commercial refrigeration, automotive glazings, and electronic displays.
Thus, there is a need for a method and materials to produce high surface area BET>100 m2/g air dried polymer aerogels by preventing the aerogel collapsing during solvent evaporation and solvent exchange steps during ambient environment solvent drying. Furthermore, there is a need for producing such aerogels that are transparent.